The present invention relates to a contact exposure apparatus wherein a member is exposed to a pattern of another member while they are in close-contact with each other, more particularly, the apparatus wherein a difference between vacuum pressure to one of the members and to the other is utilized.
Conventionally, the apparatus of this type is often used with a so-called mask aligner. FIG. 1 shows an example of such an apparatus, wherein an expansible and closed space or chamber 6 is formed between the two members which, in this case, a mask 1 and a wafer 3. The space is evacuated so that the mask 1 is bent or curved toward the wafer 3 by the atmospheric pressure exerted to the backside of the mask 1 to bring them into close-contact with each other. Since however, the gases existing in the space between the mask 1 and the wafer 3 is quickly evacuated to a high vacuum required for the desired close-contact from the atmospheric pressure, the conventional apparatus involves the following drawbacks.
Firstly, the close-contact therebetween is degraded. In the prior art apparatus, the entire surfaces of the mask 1 and the wafer 3 start contacting substantially simultaneously at all points. Then, the gases in the middle of the wafer 3 can not escape, tending to be captured there. This tendency is remarkable, particularly when the wafer 3 is processed after it has been subjected to some of many semiconductor manufacturing process steps resulting in deteriorated flatness of its surface. The gases thus captured degrade the close contact. When this occurs, the light can diffract in the fine space between the mask 1 and the wafer 3 created by the captured gases, which results in poor resolution. Thus, in the contact type exposure apparatus, the poor close-contact directly leads to poor pattern exposure. In order to vent the captured gases, the conventional apparatus is operated in such a manner that the pattern exposure operation is carried out with a delay of time after they are contacted. This, however, has been a cause of preventing the increase of throughput, that is, the number of wafers processed per unit time.
The second drawback is an increase in a pitch error. In the above described apparatus, when the mask 1 and the wafer 3 are to be contacted to each other, they are supported with a proper space therebetween, and thereafter, the closed space 6 formed therebetween is evacuated so as to allow the atmospheric pressure exerted to the backside of the mask 1 to curve the mask 1 toward the wafer 3, thus bringing them into close-contact with each other. In doing this, the initial setting of the gap or clearance between the mask 1 and the wafer 3 has a significant influence to the closeness of the contact.
More particularly, when the gap is small, the amount of curve of the mask 1 is small so that the closeness in the marginal areas is better and that the so-called pitch error is smaller. The pitch error is an error in which the mask pattern is projected slightly out of alignment with the position where it is to be projected, because the pattern of the mask 1 is expanded due to the curve of the mask 1. On the contrary, as explained hereinbefore, the closeness in the central area is rather poor because of the fine gaps created.
When, on the other hand, the amount of the gap is large, as shown in FIG. 2, the mask 1 is greatly curved, so that the contact between the mask 1 and wafer 3 starts at the central area, and the contacted area extends gradually toward marginal areas with the increase of the vacuum. Therefore, the occurrence of the captured gases in the neighborhood of the center of the mask 1 and the wafer 3 is mitigated, and therefore, the closeness of contact in the central area is increased. However, since the mask 1 is significantly curved toward the wafer 3, the pitch error is larger in this case.
Therefore, in the prior art apparatus, when the gap is increased in order to provide a better closeness of contact between the two members, such as the mask 1 and the wafer 3, a larger pitch error results; whereas if the gap is reduced in an attempt to decrease the pitch error, the poor closeness of contact results. Thus, it involves contradictory drawbacks.